iio: cros_ec: Remove unused variables

Fix gcc warnings about variable 'ec_device' being set but not used
in these files:
common/cros_ec_sensors/cros_ec_sensors.c:194:25
light/cros_ec_light_prox.c:184:25

Signed-off-by: Paolo Cretaro <paolocretaro@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>